Modern day airfoil assemblies such as helicopter rotor blades, flaps, ailerons, and the like, such as those shown in U.S. Pat. No. 5,320,494, include internal components which are structural support components, such as rotor blade spars; airfoil-shaping honeycomb components; and the like. These airfoil assemblies include an external skin which forms an envelope for the airfoil, and is typically made of a varying number of plies of fiberglass, or graphite impregnated with a resin matrix, generally epoxy, which is referred to in the industry as "prepreg". The internal airfoil components are preassembled and then are positioned in a bonding assembly which serves to locate and bond the external skin to the internal components of the airfoil. The bonding assembly includes cooperating mold parts including a steel base member having an internal cavity that conforms to the desired configuration of the upper or lower surface of the airfoil; and a caul plate member which forms the upper closure of the bonding cavity. The caul plate member can take the form of a steel or aluminum cover for the bonding cavity during the bonding operation.
The use of rigid upper and lower components of the mold is acceptable in most cases; however, when an air foil having complex and pronounced surface contour changes is being formed, the rigid caul plates may not be useful since such airfoils must be formed at higher pressures so as to ensure that the skin follows the machined contours of the core without leaving voids between the airfoil skin and the airfoil core. We have experimented with a compound caul for high pressure molding of airfoils, which compound caul included a central flexible component and peripheral rigid elements. The flexible component was formed from silicone rubber, and the peripheral rigid elements were formed from aluminum spars or beams. We found that the rigid aluminum caul elements resulted in the formation of undesirable peripheral areas in the molded airfoil which contained voids wherein the skin was not bonded to the core. We believe that these voids are the result of the inability of the rigid caul elements to adapt to areas of the core wherein pronounced surface contour changes occur.
We have also experimented with a caul which was made solely from the silicone rubber material. When such a flexible caul was used, there were no voids between the skin and core, but the edges of the core were undesirably deformed, i.e., crushed, by the molding pressure, and the inability of the flexible material to protect the edges of the core against crushing. What is needed is a caul component which can conform to the contours of the core, while at the same time protecting the edges of the core against crushing. We have discovered that a wire mesh which is commonly used as a conveyer belt has the necessary properties to be used as an airfoil caul component in combination with a wholly flexible component of silicone rubber, or some similar elastomer.